Typically, a solid oxide fuel cell (SOFC) employs a solid electrolyte of ion-conductive solid oxide such as stabilized zirconia. The electrolyte is interposed between an anode and a cathode to form an electrolyte electrode assembly. The electrolyte electrode assembly is interposed between separators (bipolar plates). In use, generally, predetermined numbers of the electrolyte electrode assemblies and the separators are stacked together to form a fuel cell stack.
As the fuel gas supplied to the fuel cell, normally, a hydrogen gas produced from hydrocarbon raw material by a reformer is used. In general, in the reformer, a reformed raw material gas is obtained from hydrocarbon raw material of a fossil fuel or the like, such as methane or LNG, and the reformed raw material gas undergoes, e.g., steam reforming to produce a reformed gas (fuel gas).
The operating temperature of the fuel cell of this type is relatively high. Therefore, an exhaust gas therefrom containing a fuel gas and an oxygen-containing gas that have been consumed in the power generation reaction are hot. Thus, it is desired to effectively utilize the exhaust gas. In this regard, for example, a fuel cell system disclosed in Japanese Laid-Open Patent Publication No. 2006-024430 is known.
As shown in FIG. 7, this fuel cell system includes a solid oxide fuel cell 1a, a heat exchanger 2a for performing heat exchange between the exhaust gas from the solid oxide fuel cell 1a and water, a hot water tank 3a for storing water, a circulation pipe 4a connecting the bottom of the hot water tank 3a and the heat exchanger 2a and connecting an upper portion of the hot water tank 3a and the heat exchanger 2a to circulate water between the hot water tank 3a and the heat exchanger 2a, a circulation pump 5a provided in the circulation pipe 4a for forcibly circulating the water, temperature detectors 6a, 7a for detecting the temperature of water at the inlet and the outlet of the heat exchanger 2a, and a control device 8a for controlling the output of the circulation pump 5a such that the temperature of the water at the outlet of the heat exchanger 2a becomes higher than the temperature of the water at the inlet of the heat exchanger 2a by a predetermined temperature.
Further in a fuel cell system disclosed in International Publication No. WO 2007/052633, as shown in FIG. 8, a solid oxide fuel cell 1b, a heat exchanger 2b for performing heat exchange between an exhaust gas from the solid oxide fuel cell 1b and water, a hot water tank 3b for storing water, a circulation pipe 4b for circulating water between the hot water tank 3b and the heat exchanger 2b, a circulation pump 5b provided in the circulation pipe 4b, and a control device 6b for controlling the fuel utilization ratio during power generation of the solid oxide fuel cell 1b in correspondence with the amount of hot water to be used are provided.
Further, in a fuel cell system and a cogeneration system disclosed in Japanese Laid-Open Patent Publication No. 2003-187843, a fuel cell unit, an exhaust gas combustion unit, and a first heat exchanger unit are provided. The fuel cell unit is connected to electric loads. The fuel cell unit generates fuel cell electrical energy by consuming a fuel gas and an oxygen-containing gas, and supplies the electrical energy to the electric loads. The exhaust gas combustion unit combusts the fuel gas and the oxygen-containing gas consumed in the fuel gas unit to produce a combustion exhaust gas. The first heat exchanger unit recovers heat from the combustion exhaust gas through a heat medium.
The fuel cell unit is operated continuously at a predetermined temperature or more so that electrical energy generated in the fuel cell can be supplied to the electric loads even if no electrical energy is required for the electric loads. The heat is supplied to heat utilization equipment which utilizes the heat medium.